The terminal differentiation events of erythropoiesis do not occur in isolation, but rather within a specialized niche known as the erythroblastic island Adhesive macrophage-erythroid cell interactions are important for its integrity, which efficiently aid the red cell maturation progression from erythroblast to enucleated reticulocyte. Expression of specific adhesion molecules and nuclear condensation effectors each are critical for this process. However, how expression of the genes encoding these molecules is coordinated is not established. EKLF/KLF1 (erythroid Kr?ppel-like factor) plays a global role in erythropoiesis by integrating transcriptional and epigenetic signals at specific loci. As a result, it merits consideration for performing such a role at late stages of differentiation. Such a conjecture is supported by experiments in differentiating murine embryonic stem cells, identification of relevant EKLF targets, its early and localized expression during development, deficiencies exhibited by EKLF-null fetal liver cells, the surprising expansion of EKLF expression pattern, and the red cell phenotype in a subset of congenital dyserythropoietic anemia patients. These form the basis for the following aims: 1-Investigate the role of EKLF in erythroblastic island formation and integrity during early development and in the adult 2-Elaborate on EKLF's unexpected role in the island macrophage 3-Utilize a proliferation/differentiation system of primary erythroid cells to address EKLF's ability to coordinate nuclear maturation events. This proposal is designed to test the hypothesis that EKLF plays a coordinating role in two processes that are intimately interconnected within the context of the erythroblastic island niche: adhesive erythroid-macrophage/cell-cell interactions, and enucleation of the red cell. Our studies build on observations made in primary or minimally manipulated cells that will be aided by in vivo assays and EKLF rescue systems. Our proposed experiments raise and will address the exciting idea that EKLF not only plays an intrinsic role in establishing the proper gene expression patterns in the red cell within the erythroblastic island, but that it additionally affects this process by an extrinsic mechanism based on its unanticipated expression within the island macrophage. Understanding these basic mechanisms will ultimately aid in the design of culture systems that enable efficient expansion and enucleation of human cell sources for clinical use.